The invention generally relates to the manufacture of bipolar transistors. More specifically, the invention relates to a method of manufacturing a plurality of bipolar transistors on a substrate that are insulated from each other. In this regard, vertical bipolar transistors integrated in a substrate must be electrically insulated from one another.
Bipolar transistors are often manufactured by producing a buried layer (also referred to as a buried collector) in a substrate and subsequently, surface-wide producing an epitaxial layer thereover. The substrate thereby is configured to be of one conductivity type and the buried layer and the epitaxial layer are doped to be of the opposite conductivity type. The bipolar transistor then is constructed in the buried layer and in the epitaxial layer.
Additionally, insulation is produced between neighboring transistors and the insulation between neighboring bipolar transistors must extend deeply enough into the substrate so that the epitaxial layer and the buried layer are separated.
It is known (see, for example, Braekehann et al., ISSCC Dig. Techn. Papers (1977), page 108) to insulate neighboring bipolar transistors from one another by utilizing inhibited pn-junctions. This insulating technique, however, utilizes a great deal of area.
In another insulation technique, the "recessed oxide" process, oxides are introduced and recessed in the substrate, this being known, for example, from A. W. Wieder, Siemens Forschungs-und Entwiklungsberichte, Vol. 13 (1984), pages 246 ff. Given unaltered lithography, the area requirement of the insulation can thereby be reduced to approximately one-third.
A further development of the "recessed oxide" process (see H. Kabza et al., IEEE Electr. Dev. Lett., Vol. 10 (1989), page 344) provides for the introduction of a self-aligned sidewall passivation. In this technique, a planar insulation is realized with noticeably reduced subsequent fabrication steps. Due to under-oxidation and growth suppression of oxides in narrow lithography gaps, however, this technique cannot be arbitrarily miniaturized. Scalability therefore is limited with this technique.
H. B. Pogge, in IEEE BTCM'90, Conf. Proc., 1990, page 18 and E. Bertagnolli et al., in IEEE BCTM'91 Conf. Proc., 1991, page 34, disclose that bipolar transistors can be insulated from one another by providing deep trenches filled with insulating material that surround the transistors. This insulating technique needs no complex subsequent fabrication steps and, therefore, is suitable in the further scaling of bipolar transistors. Insulation regions that define the position of the active transistor region composed of an emitter, base and collector and that define the position of a collector terminal are produced at the surface of the substrate by means of a LOCOS oxidation.
This technology requires noticeably greater process complexity as compared to that of prior technologies. In this regard, approximately 30% more process steps and an additional mask level for the insulation region are required.
The additional mask level is necessary in order to manufacture a substrate contact at the transistor side. This ensues by masked implantation into the epitaxial layer and, potentially, into the buried layer, so that the same conductivity type as in the substrate is produced in the region of the substrate contact.
The temperature stress in the LOCOS oxidation in this process sequence is so high that, first, the buried layer diffuses approximately 0.3 .mu.m deep into the epitaxial layer and, thus, the nominal thickness of the epitaxial layer is reduced by this amount, and that, second, the thermal load cycles limit the yield. Further, the dopant profile of the buried layer is flattened due to the temperature stress, this leading to a loss of performance and scalability of the transistor profiles.
Since the positions of the active transistor regions and of the collector terminal are defined in this process by a LOCOS oxidation, the distance between collector terminal and active transistor region cannot be made arbitrarily small because of the under-oxidation (bird's beak) occurring in the LOCOS oxidation.